The present invention relates to a combustion chamber of an engine with a direct fuel injection system, and more particularly to a combustion chamber configuration of the engine for promoting stratified combustion.
In a direct fuel injection system, a rich air-fuel mixture is injected from an injector near a spark plug while dispersion of the fuel is suppressed. The mixture is ignited with the spark plug and continuously burned with flame propagation to achieve the stratified combustion.
In order to improve scavenging efficiency in a light engine load range of a two-cycle engine and to prevent the fuel from escaping through an exhaust port, the direct fuel injection system has been proposed. In the system, air is only induced into the cylinder by a scavenge pump through a scavenge port, and the fuel is directly injected into the cylinder by a fuel injector after closing the scavenge port. In such a system, the fuel injector and the spark plug are disposed in the combustion chamber of the cylinder.
In a four-cycle engine, engine load is controlled by controlling the fuel quantity from the injector, thereby reducing the throttle loss of intake-air. Thus, the air-fuel ratio becomes lean to improve thermal efficiency and fuel consumption. Therefore, it is desirable to achieve the strafified combustion in a wide operational range of the engine. However, if the mixture is burned in a stratified mode in a heavy engine load range, the air can not be sufficiently used and hence large engine power does not occur. Therefore, it is necessary to change the combustion mode to a homogeneous mixture combustion.
In order to ensure a smooth transfer of the combustion mode from the stratified charge to a homogeneous mixture, it is necessary to properly determine a position and direction of the injector, the spark plug and a gap of the plug, the flow of injected fuel, a fuel injection rate, and a fuel injection timing corresponding to the ignition timing.
Conventional engines having the direct fuel injection systems have been proposed as follows.
First, "Internal Combustion Engine" (Nainen Kikan), Vol. 25, Oct., 1986, (Pages 26, 27, 29 and 31) discloses combustion methods of FORD PROCO, TEXACO TCCS, MAN-FM and VW in which a rich air-fuel mixture is directed toward the gap of the spark plug by swirling the mixture.
Second, Japanese Patent Application Laid-Open 60-1322, 60-1323 and 61-234222 disclose system, each of which has a sub-combustion chamber provided beside a main combustion chamber. The injector and the spark plug are provided in the sub-chamber so that the fuel injection, ignition and stratified charging are performed in the sub-chamber.
Third, Japanese Patent Application Laid-Open 62-147012 discloses a system in which a cavity is formed in a piston crown and the fuel is injected and ignited at a top dead center.
In the first conventional system, the cycle of the swirl largely fluctuates in accordance with the engine operating conditions by measuring with a laser flow meter. It is difficult to ignite the fuel in any engine operating condition. In a heavy engine load range, the fuel is not equally dispersed due to the swirl so that the homogeneous mixture combustion is not obtained. Consequently, large engine power in the heavy load range can not be expected.
In the second system, the charge stratification is promoted at light engine load. However, fuel consumption is increased because of the throttle loss in the passage between the sub-chamber and the main chamber. Further, at heavy engine load, air is not sufficiently used so that the engine does not produce a large engine power, but it has disadvantages such as a generation of smoke. If such a system is used for the two-cycle engine, scavenging cf the sub-chamber becomes insufficient. Accordingly, the burned gas remains in the sub-chamber, which means incomplete combustion.
In the third system, the ignition timing must be determined before the top dead center due to ignition delay. However, since the chamber is formed on the piston crown, the ignition timing and the fuel injection timing can not be properly controlled. In addition, such a system can not be used for the two-cycle engine because of insufficient scavenging.
Japanese Patent Application Laid-Open 62-32214 discloses such a two-cycle engine. The two-cycle engine has a spherical cavity formed in a roof of the combustion chamber, positioned eccentrically with the respect to the axis of the cylinder. The cavity promotes turbulence of the fresh air in the vertical direction to form a vertical swirl. The injected fuel is mixed with the turbulent air, which is carried toward the spark plug. A fuel injector and a spark plug are provided at a cavity top.
Since depth of the cavity is considerably large (0.55-0.77 times the diameter of the cylinder), a small amount of fresh air flows into the cavity. Thus, the burned gas can not be sufficiently discharged so that incomplete combustion may occur because of the insufficient scavenging. Furthermore, the spark plug is provided in the cavity, and the flame propagation is delayed at heavy engine load, causing knocking of the engine. In addition, since an axis of the cavity is located off the axis of the cylinder, the radius of the vertical swirl becomes small. As a result, while the exhaust and scavenge ports are opening, the scavenging flow is insufficient to reach the end gas. Thus, the burned gas remains in the cylinder, causing the incomplete combustion and knocking.
Since the injector provided in the cavity is also located off the axis of the cylinder, the fuel injected from the injector adheres to the inside wall of the cylinder at heavy engine load, which may cause insufficient dispersion of the fuel. Thus, combustion efficiency is reduced.